The present invention generally relates to digital copier for recording image data representative of a document by digitally processing the image data and, more particularly, to a digital copier capable of forming a blank area in a leading edge portion of a paper sheet to prevent the sheet from wrapping around a fixing roller.
It is a common practice with a copier implemented by an electrophotographic process to charge the surface of an image carrier in the form of a photoconductive element, expose the chargeded surface to image data representative of a document to electrostatically form a latent image on the element, develop the latent image, transfer the resultant visible image to a paper sheet, and then fix the image transferred to the paper sheet. To fix the image on the paper sheet, it has been customary to use a fixing device having a fixing roller in which a heater or similar heat source is incorporated and a pressin roller. The paper sheet carrying the image thereon is transported to the fixing device and, after the image has been fixed by the device, driven out of the copier.
When a paper sheet carrying an image in a leading edge portion thereof is transported to a nipping region between the two coactive rollers of the fixing device by a belt, for example, the leading edge portion adheres to the surface of the fixing roller due to the pressure being exerted by the pressing roller on the fixing roller and the heat being generated by the heat source. Then, the paper sheet wraps round the fixing roller and cannot be driven out of the copier, thereby jamming the fixing device. Especially, when the image in the leading edge portion of the paper sheet is a solid black image, for example, it is heated by the heater at the nipping section between the rollers with the result that the developer forming the image melts. Although the melted developer solidifies when the paper sheet moves through the nipping section, it contracts to some extent so that the leading edge portion of the paper sheet is curled to coil round the fixing roller. So long as the curl is not noticeable, a separating pawl having a tip remaining in contact with the fixing roller will successfully separate the paper sheet and will prevent it from wrapping round the roller. However, when the solid image in the portion of interest of the paper sheet has a substantial area, the curl is noticeable. This, coupled with the fact that the developer plays the role of adhesive sticking the paper sheet to the fixing roller, the pawl fails to separate such a portion of the paper sheet from the roller because the force urging the pawl against the roller is only weak enough to prevent the pawl from damaging the surface of the roller.
In the light of the above, it has been customary to form a blank area in a leading edge portion of a paper sheet so that the sheet may be prevented from wrapping round the fixing roller. With a prior art analog copier, two different approaches are available for forming a blank area in a leading edge portion of a paper sheet. One of them is to cause a discharge lamp or similar erasing means to erase, before image transfer, a latent image formed on a photoconductive element and lying in a leading edge portion of an image area which is associated with the size of a paper sheet. The other is to delay the operation of an image transferring device relative to a leading edge portion of a developed image formed on a photoconductive element so as not to transfer the image to a leading edge portion of a paper sheet.
Assume that the erasing means scheme is applied to a positive-positive (P/P) copier. Then, a charging unit deposits a negative charge, for example, on a photoconductive element. In this condition, the photoconductive element is exposed imagewise so that a latent image is electrostatically formed on the element in the area other than the exposed area, i.e., the area where the negative charge has been dissipated. A developing roller supplies a positively charged toner, or developer, to the latent image to develop it. Specifically, an image area is left on the photoconductive element as a high potential area. Hence, when the image area is illuminated, its image potential is erased to leave a blank area on a paper sheet.
In the case of negative-positive (N/P) image formation which is customary with an ordinary digiral copier, a negative charge, for example, is deposited on the photoconductive element. Then, the photoconductive element is exposed imagewise. The developing roller applies a negatively charged toner to the resultant area on the element where the negative charge has been dissipated, producing a toner image on the element. This sets up a low potential in the image area and a high potential in the other or non-image area. Hence, when the leading edge portion of a paper sheet is discharged by a discharge lamp or similar erasing means, it turns out to be a solid black image and, therefore, aggravates the wrapping of the paper sheet round a fixing roller. Another problem with the erasing means scheme is that a paper sheet cannot be surely prevented from coiling round a fixing roller unless a substantial blank area is left on a paper sheet, due to the irregularities in the timing for starting forming a toner image, the turn-off timing of discharge processing, the register timing of a paper sheet and a toner image.
The other or delayed transfer type scheme mentioned previously executes the transfer processing in a particular transfer region within the transferring device and not in a predetermined portion of the photoconductive element. This brings about a problem that the non-transferred area varies with the amount of toner deposited on the photoconductive element and, hence, it is difficult to form a blank area accurately on a paper sheet. Specifically, the blank area is apt to vary in position in the main scanning direction. In addition, the toner remaining non-transferred on the photoconductive element increases the load on a cleaning device more than necessary. For example, when an image includes a solid black portion, it is likely that the solid black portion prevents the cleaning device from exhibiting its expected cleaning function.
Furthermore, such a prior art procedure blanks a predetermined area of a leading edge portion of a paper sheet without exception, so that even hairlines and characters lying in the predetermined area are erased despite that they do not cause the paper sheet to wrap round the fixing roller. Assume that after the photoconductive element has been exposed imagewise, the charge deposited on the element and associated with a leading edge portion of a paper sheet is erased unconditionally over 5 mm, for example, by erasing means with no regard to the content of image data. Then, blank processing is automatically executed even with hairlines and similar image portions whose image area density (total amount of black pixels in overall area; less than 10% in ordinary documents) is less than 2% or 3%, despite that such images do no cause a paper sheet to wrap round the fixing roller at all. As a result, even the characters or similar image portions are erased to produce an incomplete reproduction.
The wrapping of a paper sheet round the fixing roller is effected not only by the condition of a leading edge portion of a paper sheet but also by the thickness, material and straining direction of the paper sheet. For example, a thin paper sheet which is little elastic is apt to easily wrap round the fixing roller even if its image area density is low. Conversely, a thick paper sheet having sufficient rarely wraps round the fixing roller although its image area density may be high.
Especially, a digital copier having a N/P reversal capability cannot surely prevent a paper sheet from coiling round the fixing roller when implemented by the prior art method which unconditionally blanks a paper sheet over 5 mm, for example. This is because even when the image area density of a positive document is as low as 2% to 3%, an image produced by reversal is equivalent to a document whose image area density is as high as 97% to 98%. Conversely, when use is made of a negative document the image area density of which is 95% to 98%, the N/P reversal reduces the image area density to only 2% to 3%. Then, despite that blanking a leading edge portion of a paper sheet is not necessary, the leading edge portion is also blanked unconditionally over 5 mm, for example.
Moreover, since irregularities in the operation timing of the erasing means has to be taken into consideration, the required amount is erasure is greater than a minimum necessary amount.